1. Field of the Invention
The present invention relates to a method and apparatus for polishing a workpiece, and more particularly to a method and apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused on by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surface of semiconductor wafers flat for photolithography. One customary way of flattening the surface of semiconductor wafers is to polish them with a polishing apparatus.
Conventionally, such a polishing apparatus has a turntable, and a top ring which exerts a constant pressure on the turntable. An abrasive cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the abrasive cloth and clamped between the top ring and the turntable. The semiconductor wafer is securely fixed to the lower surface of the top ring by wax, a pad or a suction so that the semiconductor wafer can be rotated integrally with the top ring during polishing.
However, in the conventional polishing apparatus, since the semiconductor wafer is fixed on the lower surface of the top ring, small convex surfaces are formed on the semiconductor wafer to be polished by dust particles interposed between the semiconductor wafer and the lower surface of the top ring. The convex surfaces on the semiconductor wafer tend to be overpolished, thus forming a plurality of thin spots, so-called bull's-eye. In order to avoid formation of the bull's-eye, dust particles must be perfectly removed by washing the lower surface of the top ring, or an elastic material such as wax or a pad must be interposed between the semiconductor wafer and the lower surface of the top ring so as not to form the convex surfaces by dust particles.
However, it is difficult to remove dust particles perfectly by the washing process and to judge whether dust particles are perfectly removed or not. Further, to attach the semiconductor wafer to the top ring using wax is troublesome and time-consuming.
Furthermore, in case of interposing an elastic material such as a pad between the semiconductor wafer and the lower surface of the top ring, repeated pressure applied to the elastic material makes the service life of the elastic material relatively short.